Wike,
Something that is mentioned in the other replies, but needs to be more clearly stated.
Brake fluid cause 2 problems
1 It removes Paint
2 It is hydroscopic, attracts and combines with water
When brake fluid leaks into the booster it strips the protective coating of paint on the inside of the booster. Once the paint is gone and the coating of brake fluid mixes with moisture in the air. The inside of the booster will begin to rust. It won't take long for the metal to pit. Ultimately leading to the destruction of your booster from the inside out.
The same thing can happen to the bottom outside of the booster. If your old master cylinder was leaking at the bottom of the seal where the booster rod pushed the master cylinder rod into the MC. The fluid may have also leaked onto the outside of the booster, dripping down the underside. The paint will be gone, metal exposed to the ravages of hydroscopic brake fluid.
I agree with the comments from the others that you need to have the booster taken apart and fixed ASAP. If you are concerned that this is beyond your skills or do not have the tools Brosky mentioned. Ship it to the experts.
I'm in the middle of replacing my master cylinder and upgrading my brakes. My MC began to leak just a little at the bottom of the seal mentioned above. I suspect the initial leakage was very slight but as fluid accumulated it corroded the MC allowing more fluid to escape. Some began to drip down the bottom side of the boosted. It removed the paint and surface rust began to appear. I removed the MC and Booster. I found no evidense of fluid entering the booster. I thoroughly cleaned the outside of the booster with brake cleaner followed by several scrubbings with dish soap. Then removed all the paint with a wire wheel, wire brushed all surface rust until it was gone. Used a phosphoric acid gell (Aluminum jelly) to acid wash the surface twice. (To provide the paint with a freshly etched surface for better adhesion). Dried and painted with VHT Caliper paint, dried over night, then baked it for an hour at 175
Note: even with the scrubbing and hosing down at various steps I never saw any water come from inside the booster. I feel it was safe to assume the seals on the booster are intact and prevented leakage into the body. BTW. I've used the VHT caliper paint on other vehicles. Brake fluid does not affect it.
As fo my Master Cylinder I opted to have it sleeved and rebuilt by White Post. The price was more than the cost of an economy MC and less than the cost of a premium MC. I sent it to them on a Monday. It was back the following Monday.
My reason for the sleeving is that as the seals get old and start to weep brake fluid they will not attack the surface where the seal is made. Brass should not corrode like the original cast iron of the MC. If I'm correct I should not have to buy a new MC in the future. Only install a rebuild kit as the current one begins to age. WP recommends changing the brake fluid every few years. This should minimize the amount of moisture absorbed by the fluid.
White Post provides a writtten lifetime guarantee for their materials and workmanship. I'm sure this does not include the rebuild kit. I have no financial interest in White Post. I do have a considerable financial interest in my TR6! But I will say their service was excellent!
BOBH